LOL. See 0:34~0:38 where he explains his accu-punch.
Clearly demonstrating his "accu-punch" landing with last 3 knuckles.
From wiki, he learned his accu-punch from TKD master Jhoon Goo Rhee.
To my knowledge, most schools of TKD teach punching with the 1st two knuckles. I also read in some biography (or was it Blackbelt magazine) he was a great fan and supporter of karate, that would explain his developed 1st two knuckles.